Cathode ray tube apparatus



Oct. 11, 1938. M. BOWMAN-MANIFOLD ET AL 2,132,933

, CATHODE RAY TUBE APPARATUS Original Fil'ed Oct. 25, 1935 2Sheets-Sheet 1 l VENTQIS MICHAEL BOWMANMANI ALAN DOV/ BLUMLEI ATTORNEYCATHODE RAY TUBE APPARATUS 2 SheetsSheet 2 OriginaLFiled Oct. 25, 1935INVENTOR Nw Y AL E MB N mgm B w L0 5% A HN Patented Oct. 11, 1938 UNITEDSTATES PATENT OFFICE CATHODE RAY TUBE APPARATUS Original applicationOctober 23, 1935, Serial No.

46,272. Divided and this application September 3, 1936, Serial No.99,194. In Great Britain October 24, 1934 3 Claims.

The present invention relates broadly to improvements in cathode raytube apparatus and more particularly to an improved form of deflectingmeans for controlling the cathode ray beam developed therein. This is adivision of my application, Serial No. 46,272, filed October 23, 1935.

Cathode ray tubes for current and potential analysis, and for use intelevision and like systems,are provided with means for deflecting thecathode ray and for causing it to trace out a desired path on afluorescent or other screen associated with the tube. The deflectingmeans often comprise two deflecting coils, or two pairs of deflectingcoils, arranged to produce deflection of the ray in two mutuallyperpendicular directions.

In television systems for example, it is generally required that thecathode ray should trace out on the screen a number of successiveparallel lines, each slightly below the one preceding it, thearrangement being such that the area scanned is of rectangular shape. Itis accordingly required that the deflecting coils should producedeflection of the ray in two coordinate directions, the deflection inone direction taking place at a higher frequency than the deflection inthe other direction. Such a deflection of the ray is achieved inpractice, in a manner well known per se, by feeding to the deflectingcoils, or pairs of coils, deflecting currents of suitably chosendifferent frequencies. The deflecting currents are generally ofsaw-tooth wave form.

The method of scanning outlined above produces distortion if the areascanned is of other than the desired shape; consequently, the deflectingmeans must be so constructed and arranged that the area scanned is ofthe desired shape and dimensions. For example, if it is desired to scana rectangular area on the screen, using deflecting currents of saw-toothwave form, it must be arranged that for a given change of current in onedeflecting coil or pair of coils, the distance moved through by thecathode ray on the screen is independent of the initial position of thescanning spot where the cathode ray strikes the screen.

If the deflection of the beam in either coordinate is not a linearfunction of the deflecting current, that is, if it is not independent ofthe initial position of the scanning spot, the following undesirableresult will occur; if the beam is converging while it passes through thedeflecting field, difierent portions of the beam will be dif ferentlydeflected, and sharp focus will be lost,

while even if the beam be assumed to be infinitely thin, distortion ofthe picture will arise, as already explained.

It has proved in practice very difiicult to provide deflecting coilscapable of operating without introducing distortion of the kinddiscussed above.

A further difiiculty which has been encountered in designing deflectingcoils for cathode ray tubes will now be considered. Such tubes usuallycomprise an envelope having a frustro-conical portion joined to acylindrical portion, and the coils have usually been mounted on thecylindrical part of the envelope of the tube; for convenience, this partof the envelope will be referred to as the neck of the tube.

When a set of coils is required to deflect the cathode ray through largeangles, it is necessary to make the diameter of the neck of the tuberelatively large, in order that the deflected beam shall not hit theneck but shall pass unobstructed into the frustro-conical portion of thetube. For large angles of deflection, therefore, it is necessary to makethe diameter of the coils relatively large; inefficient coils are thusobtained, not only on account of the large diameter, but also since itmay be necessary to make the length to diameter ratio of the coilsrelatively small; with a coil of which the length to diameter ratio issmall, the uniformity of the field obtained is found to be poor.

It is one of the objects of the present invention to provide a cathoderay tube apparatus wherein the deflecting coils are so constructed andarranged that the deflection of the ray in each co-ordinate directiondepends only on the deflecting current fed to the coils, so thatdistortion of the area scanned due to the deflecting coils is avoided.This will be realized if the set of coils for deflecting the ray in eachdirection produces a magnetic field which, in every plane at rightangles to the axis of the cathode ray tube, is of uniform strength anddirection over the area which the cathode ray beam traverse.

Another object of the invention is to provide a cathode ray tube whereinthe deflecting means produce a magnetic field substantially only whereit is required, without the expenditure of magnetizing energy inproducing a field elsewhere, thereby reducing the magnetizing energywhich is necessary to produce a field of the required flux density. Thisarrangement is adaptable to deflect the cathode ray beam through largeangles. The deflecting coils according to this invention is wound abouta space of cylindrical or frustro-conical shape, and adapted to producewithin said space a magnetic field substantially at right angles to theaxis of said space, said coil lying wholly outside the cylinder or coneof which said space fills a part, wherein said coil comprises aplurality of turns having active portions which lie close to said spaceand approximately parallel to the axis thereof, and wherein said activeportions are so arranged that when current is passed through said coil,the magnetic field set up within said space, in a plane at right anglesto the axis thereof, is substantially uniform. v

According to a feature of the present invention, a coil is wound to fillthe whole or a part of the space defined by the surfaces of twoimaginary solid figures both in the form of cylinders or conical frustraof substantially the same size and shape, of which one has beendisplaced bodily from a position of substantially complete coincidencewith the other in a direction at right angles to the direction of theaxes of the said figures through a distance short compared to thedimensions of each figure in a plane at right angles to the axisthereof, the winding being carried out in such a manner that when, inoperation, current is caused to fiow in said coil, the direction ofcurrent fiow in that part of said coil which is on one side of a planeof intersection lying substantially at right angles to the direction ofsaid displacement and containing two lines of intersection of thesurfaces of said figures is opposite to the direction of current fiow inthe part of said coil lying in the other part of said space; the coilmay be located within and close to a sheath of magnetic material whichserves as a return path for the magnetic flux.

In the preceding paragraph, the term cylinder is to be understood tomean the solid figure generated by a straight line which moves so thatit is always parallel to a fixed straight line, and so that the endstrace out a closed curve. The term thus includes solid figures ofcircular, rectangular or any other cross section bounded by a closedcurve. When the surfaces of the figures intersect one another along morethan two lines, the plane of intersection referred to in the precedingparagraph is the median plane; it will be found that whatever the shapeof the figures in crosssection, no difiiculty will be found indetermining this median plane.

The term conical frustrum is intended to cover a frustrum of the solidfigure generated by a line-which is usually straightand which moves sothat one point in it remains fixed, while any other point therein tracesout a closed curve such, for example, as a circle or a rectangle.

The axis of a solid figure, for the purpose of this specification isintended to mean a line running longitudinally through the figure, butnot intersecting the surface thereof, the axis lying substantially in amedian position; for example, the axis in the case of a right circularcylinder or frustrum is the axis of symmetry of the figure.

A coil according to a further feature of this invention comprises aplurality of turns each having two active portions arrangedsubstantially parallel to one another and to the active portions of theother turns, and is so constructed that when, in operation, currentflows in said coil, the

current per unit angle in the active parts of the turns of said coil inthe neighbourhood of a plane which contains the longitudinal axis of thecoil and cuts the lines of force inside the coil substantially at rightangles, is greater than the current per unit angle in the neighbourhoodof a plane which also contains said axis but is at right angles to saidfirst-mentioned plane. By the expression current per unit angle is meantthe current which fiows through the part of the crosssection of the coilbounded by two straight lines which intersect at the axis of the coiland make a unit angle with one another.

The longitudinal axis of the coil, referred to in the above paragraph,is a line lying within the coil in a substantially median position,which does not intersect the active parts of said turns but issubstantially parallel to said active parts.

Our invention will best be understood by reference to the accompanyingdrawings in which Fig. 1 is an explanatory curve, Figs. 2 and 3 areindicative coil formers, Fig. 4 is a cross-sectional view of a coilmounting, and Fig. 5 is a longitudinal view of a coil mounting.

Before particular embodiments of the invention are describedreferencewill be made for purposes of explanation to Fig. l of the accompanyingdrawings; the disposition of the closed curve I in this figure withrespect to axes X and Y, is exactly the same as that of the curve 2 withrespect to axes X and Y, and the distance X between the Y and Y axes issmall compared to the distance from the origin 0 to any point on thecurve l. r

. Now the distance between the curves along any line parallel to the Xaxis is constant and equal to X, and hence the area of the parallelogramenclosed by the curves l and 2 and any pair of lines, parallel to the Xaxis and separated by an infinitely small distance, is constant. Thus itwill be clear that if a line such as AB is drawn parallel to the Y and Yaxes, the area of the cross-hatched portion ABCD between the two curvesis proportional to the length AB.

Let it now be assumed that the line CDEFG represents the boundary of atunnel, of constant cross section, cut in a block of magnetic materialsuch as iron, the tunnel extending downwards into the paper. The curvesl and 2 then represent cross sections of two identical cylinders havingas axes lines drawn perpendicularly downwards into the paper throughpoints 0 and O, and a coil of wire of uniform cross section is accordingto this invention wound in the space enclosed by the surfaces of thesecylinders; the winding is carried out insuch a manner that the parts ofthe turns lying in the space between the surfaces of the cylinders runparallel to the axes of the cylinders, the arrangement being such thatwhen a current is passed through the coil, the direction of current fiowin the part of the space to the left of the plane extending verticallydownwards and containing the lines of intersection of the surfacesthrough E and G is opposite to the direction of current fiow in the partof the space to the right of that plane. There is thus a uniformdistribution of current over the whole of the space between thecylinders.

Now since the cross-hatched area ABCD is proportional to the length AB,it will be clear that the magnetomotive force around the loop ABCD isalso proportional to the length AB. The return path of lines of forcepassing from A to B comprises the path AD and BC and a path through thesurrounding iron, which will be assumed to be of zero reluctance. Thelengths AD and BC are both equal to the small displacement X and arethus small compared with AB for most values which AB may have. The-wholereturn path may thus be assumed. to be of zero reluctance. Thereluctance of the air path from A to B is proportional to the length AB,and thus the reluctance of the loop ABCD is also proportional to thelength AB. It follows then that the flux density along line AB isindependent of the length of that line. The same is true of any linesuch as AB parallel to the Y axis, and hence the flux throughout thewhole space within the coil is uniform.

In a simple case, the tunnel is rectangular in cross-section and in thiscasethe coil comprises two uniform layers of straight, parallel wiresarranged along opposite sides of the tunnel and constituting the activeportions of the coil. The iron in which the tunnel is cut constitutesthe return path for the flux, and in practice preferably comprises asheath, of any desired suitable shape, fitting closely around the coil.The active portions of the latter are connected by end portions arrangedoutside the sheath.

The construction and method of mounting of a pair of coils according tothe present invention, for use as the deflecting coils of a cathode raytube, will now be described by way of example. Reference will be made toFigs. 2 to 5 of the accompanying drawings, of which Fig. 1 has alreadybeen referred to; Figs. 2 and 3 respectively to the drawings show incross-section suitable formers for winding a pair of coils according tothe invention, Fig. 4 shows in cross-section a cathode ray tube with thecoils mounted thereon, the section being taken at right angles to thelongitudinal axis of the tube, and Fig. 5 illustrates another embodimentof the invention.

Referring to Fig. 4, the coils are mounted between the wall of the tube3 and a ring 4 of magnetic material; the inner coil is shadedvertically, and the outer is shaded horizontally. The inner coil iswound on the former shown in Fig. 2, which comprises a wooden block 5having a channel or groove cut in it and a portion 6 in the shape of ahalf of a right circular cylinder of wood. The radius r of the portion 6is the radius of the outer wall of the cathode ray tube 3, Fig. 4. Thechannel in the block 5 is cylindrical in shape, and of the cross sectionwhich is defined by describing two quarter cir-.

cles of radius T1 about centres C and C, the channel is slightlyflattened at its bottom. The portion 6 is also slightly flattened, andthe two portions are held together in the positions shown by areleasable connection (not shown). The coil is wound of insulated wireso as to fill the Whole of the space between the portions 5 and 6 of theformer, the wire being carried continuously around the portion 6, downone side and back along the other.

Two such coils are wound, and are then mounted on the tube 3 in themanner shown in Fig. 4, where references 1 and 8 designate the two coilswhich together constitute the inner coil. The coils are connectedtogether in series, or parallel, depending on the inductance desired,the arrangement being such that when current flows in one direction inthe left hand portions of the coils, it flows in the opposite directionin the right hand portions thereof.

Two similar coils 9 and I0 are wound on the former shown incross-section in Fig. 3. The portion I5 is channelled wooden block, thechannel being of semi-circular cross-section and of a radius R equal tothe inner radius of the sheath 4. The cross-section of the approximatelycylindrical portion I6 is determined as shown by two quarter circles ofradius R1.

The coils 9 and I 0 are mounted upon the coils 1 and 8, if desired withthe interposition of suitable insulating material, or of suitably shapedseparating members, and the whole coil assembly is surrounded by amagnetic sheath 4 which may be built up from semi-circular or circularlaminations. The coils 9 and ID are connected in series or parallel, andit is arranged that when current flows in one direction in the upperhalves of these coils, it flows in the opposite direction in the lowerparts thereof. The resultant field due to coils 'l and 8 is at rightangles to that produced by coils 9 and ID. The overall length of theouter coils may be made rather less than that of the inner coils, ifdesired.

The parts of the turns outside the sheath 4 pass over the wall of thetube 3, and are so arranged that they lie in planes substantially atright angles to the longitudinal aXis of the tube: the magnetic fielddue to them thus produces little or no effect on the action of thedeflecting field in the tube. The sheath may be formed by winding overthe active portions of the turns of the deflecting coils a thick coil ofiron wire; the shape of the sheath is of little importance, provided itsprovides flux-return paths of a reluctance which is negligible comparedwith the reluctance across the tube.

It is found that a coil of the form last described above, that is to saya coil wound to flll the space between the surfaces of two substantiallyright circular cylinders displaced from one another, is capable ofproducing a uniform field in the absence of an iron sheath. It is alsofound that in the absence of the iron sheath, the inductance of such acoil is approximately halved.

Referring to Fig. 5, a cathode ray tube has a glass envelope comprisinga cylindrical portion I! of about one inch diameter closed at the lefthand end, and having the other end joined to a right circular conicalfrustrum l8 2 inches in length; the frustrum l8 has a diameter at itslarger end of. 2 inches, and this larger end is joined to the smallerend of a second right circular frustrum I9 which flares at a greaterrate than the first frustrum and is closed at its larger end by an endwall 20. A fluorescent screen is mounted on this end wall.

Within the cylindrical portion l 1 is mounted an electron gun which maybe of any suitable kind, and comprises essentially a cathode andmodulator assembly 2|, a first anode 22 and a second anode 23. It isarranged that the second anode 23 lies near to the smaller end of thefirst frustrum.

Mounted upon the outer wall of the first frustrum I 8 are two pairs ofdeflecting coils indicated by reference 24; these coils are of a formvery similar to that of the coils described with reference to Fig. 4,but instead of being wound to flt a cylindrical portion of the tube,they are wound to fit a frustro-conical portion. The coils areconveniently wound on formers such as are described in Figs. 2 and 3,but differing in that the winding boundaries are not cylindrical, butfrustro-conioal. The formers are so constructed that the depth of thewinding space decreases towards the wider end, and the nature of theresulting coil is therefore such that its thickness decreases as itsdiameter increases, as shown in Fig. 5. This is necessary in order thatthe same number of turns shall be accommodated at the narrower end as atthe wider end.

A laminated iron yoke 25 having a conical hole therein may, if desired,be mounted as a snug fit over the scanning coils 24, and a cross-sectionthrough the coils in a plane at right angles to the longitudinal axis ofthe tube appears much as is shown in Fig. 4. The end portions of theturns of the coils are arranged to lie in planes substantially at rightangles to the longitudinal axis of the tube, and thus the magnetic fieldclue to these portions produces little efiect on the defleeting field.

The part of the envelope on which the coils are mounted may be of otherthan right-circular frustro-conical shapes, for example, the tube mayhave a part in the shape of a frustrum of a pyramid and the coils may bewound on this part. The coils may also be wound on a portion of the tubewhich is in part of one shape, for example, cylindrical, and in part ofanother, for example conical. Thus the envelope of the tube may comprisea cylindrical portion flaring out into a single frustro-conical portionhaving a fluorescent screen at its larger end, the coils being arrangedpartly on each of the two portions. The coils may then be wound onformers so arranged that the winding space is of constant depth alongthe cylindrical parts, and tapers along the conical parts in the manneralready described.

In an arrangement such as that last described, the inner pair of coilsonly may lie on both the cylindrical and the conical parts of the tube,the outer coils being arranged to lie over the cylindrical portions onlyof the inner coils. The end turns of the inner coils may be spread alongthe surface of the frustro-conical part of the tube, thus giving themaximum length for the outer coils and the maximum length for themagnetic yoke if one is employed.

As has already been stated, the application of the invention is notlimited to coils for use in association with cathode ray tubes;furthermore, the invention is not limited to the embodiments described.Many other uses for coils according to the invention, and manymodifications thereof within the scope of the appended claims will ocourto those versed in the art.

What we claim is:-

l. A cathode ray tube including means for developing a cathode ray beamand fluorescing means adapted to respond to the impingement of saidcathode ray beam thereon, an envelope, and means for controlling thedeflection of the cathode ray beam comprising at least one coil woundabout a solid figure the shape of which is substantially that defined bya straight line which rotates about another nonintersecting straightline, the two straight lines lying always in the same plane, forproducing within said space a magnetic field at right angles to the axisof said space, said coil comprising a plurality of turns lying whollyoutside the solid figure of which said space occupies a part, each ofsaid turns comprising active. portions lying substantially parallel tosaid axis, said active portions being so distributed that when currentflows in said coil, the magnetic field at all points in a plane at rightangles to said axis is substantially uniform, and mounted on saidenvelope so as to produce a substantially uniform field at right anglesto the axis of said tube.

2. A cathode ray tube including means for developing a cathode ray beamand fluorescing means adapted to respond to the impingement of said'cathod ray beam thereon, an envelope, a sheath of magnetic materialarranged around the surface of said envelope and spaced apart therefrom,means for controlling the deflection of the cathode ray beam comprisinga coil wound about a solid figure the shape of which is substantiallythat defined by a straight line which rotates about anothernonintersecting straight line, the two straight lines lying always inthe same plane, for producing within said space a magnetic field atright angles to the axis of said space, said coil comprising a pluralityof turns lying wholly outside the solid figure of which said spaceoccupiesa part, each of said turns comp-rising active portions lyingsubstantially parallel to said axis, said active portions being sodistributed that when current fiows in said coil, the magnetic field atall points in a plane at right angles tosaid axis is substantiallyuniform, said coil filling at least partly the space between said tubeenvelope and said sheath.

3. A cathode ray tube including means for developing a cathode ray beamand fluorescing means adapted to respond to the impingement of saidcathode ray beam thereon, an envelope, a sheath of magnetic materialarranged around the surface of said envelope and spaced apart therefrom,means for controlling the deflection of the cathode ray beam comprisinga pair of coils each wound about a solid figure the shape of' which issubstantially that defined by a straight line which rotates aboutanother nonintersecting straight line, the two straight lines lyingalways in the same plane, for producing within said space a magneticfield at right angles to the axis of said space, each of said coilscomprising a plurality of turns lying wholly outside the solid figure ofwhich said space occupies a part, each of said turns comprising activeportions lying substantially parallel to said axis, said active portionsbeing so distributed that when current flows in said coils, the magneticfield at all points in a plane at right angles to said axis issubstantially uniform, said coils filling at least partly the spacebetween said tube envelope and said sheath, so that the magnetic fieldof each of the coils is substantially at right angles to that of theother.

MICHAEL BOWMAN-MANIFOLD. ALAN DOWER BLUMLEIN.

